(a) Field of the Invention
The present invention relates to a driving apparatus for a plasma display panel (PDP) and a gray level expressing method thereof, and more particularly, to a driving apparatus for a plasma display panel and a gray level expressing method thereof that can provide an improved expression of gray level and a reduction of pseudo-contour.
(b) Description of the Related Art
Flat displays, such as a liquid crystal display (LCD), a field emission display (FED), a plasma display panel, or the like, have been developed recently. Among the flat displays, the plasma display panel has an advantage in that it has a wide visual range and the brightness and light-emitting efficiency are high in comparison with other types of flat displays. The plasma display panel is in the spotlight as a display that can be substituted for the conventional cathode ray tube (CRT), especially in the large-sized displays of greater than forty inches.
The plasma display panel is a flat display that can display characters or images with plasma generated by gas discharge, on which hundreds of thousands or millions of pixels are arranged in a matrix format according to the size thereof. Such a plasma display panel is classified as a direct current type and an alternating current type according to the structure of discharging cells and the shape of the waveform of the driving voltage applied thereto.
The direct current type plasma display panel has a shortcoming in that a current flows in a discharge space while the voltage is being applied as the electrodes are exposed to the outside while the discharge space is not insulated, and for such a reason, a resistor for confining the current has to be prepared. On the other hand, the alternating current type plasma display panel has an advantage in that the current is confined by a capacitance formed naturally and the electrodes are protected by the impact from ions during the discharge by the dielectric layer covering the electrodes, so the lifetime is longer than that of the direct current type.
FIG. 1 is a partial perspective view of the alternating current type plasma display panel. As shown in FIG. 1, scan electrodes 4 and sustain electrodes 5 covered by dielectric layer 2 and protection layer 3 are formed in parallel in pairs on glass substrate 1. A plurality of address electrodes 8 covered by insulation layer 7 are formed on another glass substrate 6. Partitioning walls 9 are formed in parallel with address electrodes 8 on insulation layer 7 between address electrodes 8, and fluorescent substances 10 are formed on the surface of insulating layer 7 and both sides of partitioning walls 9. Glass substrates 1, 6 face to each other with discharge spaces 11 between them so that scan electrodes 4 and sustain electrodes 5 are perpendicular to address electrodes 8. Discharge spaces near intersections between address electrodes 8 and scan electrodes 4 and sustain electrodes 5 that are paired with each other form discharge cells 12.
FIG. 2 shows the arrangement of the electrodes in the plasma display panel. As shown in FIG. 2, the electrodes in the plasma display panel are arranged in m×n matrix form, and more particularly, address electrodes A1–Am are arranged in a column direction and n rows of the scan electrodes Y1–Yn and the sustain electrodes X1–Xn are arranged alternately in a row direction. Discharge cell 12 in FIG. 2 corresponds to the discharge cell 12 in FIG. 1.
The driving period of such an alternating type plasma display panel consists of a reset time, an addressing time, and a sustain time according to the time flow of the change of the operation.
The reset time is the period to initialize the status of the respective cells in order to enhance the performance of the addressing operation of the cells, and the addressing time is the period to form a wall charge by applying the address voltage to the cells to be turned on (addressed cell) in order to select the cells to be turned on and not to be turned on in the panel. The sustain time is the discharge period for displaying the image actually on the addressed cells by applying sustain pulses.
As shown in FIG. 3, the plasma display panel realizes a gray level by dividing one frame (1TV field) into a plurality of subfields and then performing time-divisional control thereon. The respective subfields consist of the reset time, the addressing time, and the sustain time as described above. FIG. 3 shows the case in which one frame is divided into eight subfields in order to realize 256 gray levels. The respective subfields SF1–SF8 consist of a reset time (not shown), addressing time Ad1–Ad8, and sustain time S1–S8, and in the sustain time S1–S8, the ratio of illuminating times 1T, 2T, 4T . . . , and 128T is 1:2:4:8:16:32:64:128.
In such a situation, in order to realize the gray level of 3 for example, the sum of the discharging time is made to be 3T by discharging the discharge cells at subfield SF1 having illuminating time 1T and subfield SF2 having illuminating time 2T. The image of 256 gray levels can therefore be realized by combining the subfields having different illuminating times.
Furthermore, according to the conventional method of expressing the gray level of the plasma display panel, the number of pulses allotted to the respective subfields is determined by a multiple of the subfield weight corresponding to the sustain time as shown in FIG. 3 according to the average gray level at every frame. In other words, the number of sustain pulses is changed according to the average gray level of every frame in order to increase the contrast between the frames and simultaneously decrease the power consumption. For example, to express 256 gray levels, four times the subfield weight is employed in the case of a low average gray level in order to assign many sustain pulses, and two times the subfield weight is employed in the case of a high average gray level in order to assign a small number of sustain pulses. Therefore, the conventional method is limited in enhancing the expression of the gray level since the gray level is expressed only by increasing the total sum of the sustain by multiplying a certain number to the subfield weight determined only in consideration of the gray level irrespective of the sustain pulse number.
In addition, while a moving picture is being displayed according to such a subfield method, pseudo-contour is generated due to the visual characteristics of a person. FIG. 4 shows an example of generated pseudo-contour. When an image in which gray level 127 and gray level 128 exist adjacently is moving rightward, such a status is expressed as FIG. 4 according to the subfield arrangement of FIG. 3. In such a situation, a human recognizes the gray levels in the direction of the dashed arrows shown in FIG. 4 according to the characteristics of the visual sense of the person that follows the movement of the image. Thus, a pseudo-contour such as the gray level 255 between the positions of gray levels 127 and 128 may occur.